Current evidence supports the long-held concept that the reproductive potential of female mammals is determined by the stock of non-growing, primordial follicles available at birth. These primordial follicles are formed when mitotically active germ cells in cysts cease dividing and arrest in meiosis, after which the cysts break down and some oocytes become surrounded by a few pre-granulosa cells. A small portion of these primordial follicles leave the resting pool each day and develop into growing follicles that eventually produce viable female gametes ready for fertilization. Understanding the biochemical control mechanism for these processes is important as defects could lead to a number of infertility syndromes in humans including premature ovarian failure (POF) in which accelerated loss of follicles is thought to be a possible cause. The factors regulating the processes of cyst breakdown, primordial follicle formation, and activation of primordial follicles are largely unknown but recent advances suggest that activin might be a critical component determining follicle pool size. Activin is a member of the TGF2 superfamily that is regulated by natural antagonists including follistatin (FST). The FST gene produces 3 protein isoforms which have different biochemical properties and biological actions. To determine these actions we created a mouse model in which only the smallest FST isoform, FST288 is made (FST288-only). These mice are subfertile with reduced litter size and frequency. Interestingly, FST288-only females are born with a larger pool of primordial follicles, but these follicles are depleted at a greater rate compared to WT females, suggesting that activin regulates both follicle formation and growth initiation. The broad goal of this proposal is to determine the role of activin in regulating follicle formation, development, and loss. Our central hypothesis is that activin increases germ cell proliferation, reduces apoptosis of oocytes as they form primordial follicles, and increases the number of primordial follicles leaving the resting pool. We propose to determine the mechanism(s) whereby activin regulates follicle formation and loss in Aim 1. The quality of the additional follicles in FST288-only mice will be examined in Aim 2, while in Aim 3 we will test the hypothesis that increased rate of primordial follicle loss leads to POF in FST288-only mice. These results will provide a critical foundation upon which potential applications of activin, activin analogs, or FST antagonists might be developed for treating POF patients. The long term goal of this R21 is to establish endogenous activin and its regulation by FST as critical determinants of follicle pool size and rate of decline with age, which could be relevant for understanding and treating the pathophysiology of POF and other infertility syndromes in humans. Results from the proposed research would also support future projects to use targeted and regulatable transgenic technology to control the site and timing of activin expression that will directly test activin's reproductive roles. PUBLIC HEALTH RELEVANCE: The number of eggs that female animals and humans are born with is their entire stock for their reproductive lifetime. When this stock is depleted, the ovary stops maturing new eggs, a process known as menopause in humans. In some fertility disorders, this process occurs earlier than expected, shortening the reproductive period for these patients and leaving them with few treatment options. The research in this proposal will investigate the role of activin and follistatin in regulating both the number of eggs, and the process of maturation, which could help define the defects that lead to early menopause, and to new treatments for this disorder.